The present invention relates to position encoders, particularly linear position encoders with a cylindrical or conical geometry. The present invention is applicable to, but not limited to, fluid level sensors, particularly for use in fluid dispensing systems. The present invention is particularly useful in environments subject to electromagnetic interference.
Generally, many types of non-contact linear position encoders are known. These include arrangements in which magnetically sensitive windings are mounted on a stationary element which is positioned relative to a moving element arranged to produce electromagnetic fields that couple with the stationary element. The degree of coupling is dependent upon the relative position of the movable element to the stationary element, thus the position of the movable element can be determined from the signals coupled into the windings of the stationary element.
A different type of magnetic coupling arrangement is disclosed in WO95/31696. This discloses a linear position encoder having a support upon which a pair of quadrature windings and an excitation loop are mounted. The windings are arranged to have a sinusoidal magnetic sensitivity characteristic along the length of the support. Mounted on a movable element there is a resonant circuit comprising a coil and capacitor that can magnetically couple with the windings. The excitation loop is used to produce a substantially uniform electromagnetic field that excites the resonant circuit. The resonant circuit itself then produces electromagnetic fields that induce voltages in the windings. The voltages induced are dependent upon the position of the resonant circuit within a spatial period of the windings. This disclosed position encoder has an advantage that no connections need to be made to the moving element, namely the resonant circuit, and also has an advantage that the windings are balanced with respect to far-fields, hence reducing its susceptibility to electromagnetic interference.
WO95/31696 further discloses the use of the linear position encoder in fluid flow metering applications. This is achieved by mounting the resonant circuit on a movable element which is in the form of a float. However, limitations are imposed on the suitability of the disclosed encoder to such use due to the basic geometry and operation of the disclosed device. For example, the float containing the resonant circuit must be prevented from rotating, or any rotation must be monitored and compensated, involving further resonant circuits arranged with their axes orthogonal to the original resonant circuit.
The present invention employs the separate resonant circuit approach of WO95/31696, but provides a different geometry of excitation and phase quadrature windings which alleviates the above described disadvantages. The excitation windings, phase quadrature windings and winding in the resonant circuit are arranged so that the magnetic fields involved in the operation of the encoder are substantially parallel to each other and the direction of movement of the movable element. Consequently, the effect of rotation of the movable element or lateral movement of it relative to the excitation and phase quadrature windings is removed or at any least reduced.
In one aspect the present invention provides a position encoder comprising first and second members arranged to move relative to each other in a measurement direction; the first member comprising a winding arrangement having a plurality of substantially closed planar loops whose plane is substantially orthogonal to the measurement direction, and arranged so that EMFs induced in different loops by interference are at least to an extent opposed to each other; wherein the second member comprises an electromagnetic field generator or sensor which is electromagnetically coupled to said winding so that upon generation of a magnetic field by said generator or upon application of an excitation signal to said winding, there is induced in said winding or in said sensor respectively, an output signal dependent upon the relative position of said first and second members in the measurement direction.
In a further aspect the present invention provides a position detector in which a magnetic field generator and a sensor winding are arranged so that they can move relative to each other along a path, and wherein the magnetic field generated by the magnetic field generator is substantially parallel to the path, the sensor winding having induction loops arranged substantially orthogonal to the field generated by the magnetic field generator, and arranged in a spaced apart form so that movement by the magnetic field generator along the path provides a varying level of induction in the loops that varies according to the position of the field generator within the spacing between consecutive loops.
In a further aspect the present invention provides a position encoder comprising a winding arranged to generate electromagnetic fields in a direction substantially parallel to the direction along which the position is to be measured, the winding comprising loops of opposing sense spaced apart so that the field varies between loops along the movement direction, and a movable member comprising means for generating an electromagnetic field in response to an electromagnetic field received from the winding, the size of the electromagnetic field generated being dependent upon the relative position of the member and the spacing distance between the loops, and a further winding for receiving the varying electromagnetic field from the movable element arranged to provide an output signal dependent upon the level of the electromagnetic field received from the movable member.